1. Field of the Invention
The present invention relates to an image processing apparatus which reads an image on an original and outputs the digital image data having been read as binary data after carrying out image processing thereon. More specifically, the present invention relates to an image processing apparatus used for a stencil-making apparatus (a digital printer) which carries out perforation on thermosensitive stencil paper, or to an apparatus (a digital copier) which copies on paper or thermal paper a latent image formed on a photosensitive material by using electrophotography.
2. Description of the Related Art
When an original in which binary images such as characters or drawings and tone images such as photographs coexist is output as binary data by using an image processing apparatus such as the one described above, it is necessary to obtain an optimal result that the binary image areas are binary-converted to either a maximal density value or a minimal density value by using an appropriate single threshold value, and density conversion to an appropriate density is carried out for the tone image areas by considering characteristics of input/output devices. For this purpose, it is necessary to judge whether each portion of an image is a binary image area or a tone image area and to separate the binary image areas from the tone image areas.
To tell a binary image area from a tone image area, several methods have been used. One method divides an image into blocks of n.times.n pixels and judges on each block based on the result of feature extraction carried out thereon (see Japanese Unexamined Patent Publn. No. 3(1991)-153167, for example) (hereinafter this method is called "method A"). Another method carries out the feature extraction on a target pixel and its surrounding pixels and judges on each pixel by using the feature extraction result (see Japanese Unexamined Patent Publn. No. 1(1989)-227573, for example) (hereinafter this method is called "method B").
In method A, since judging is carried out on each block, there are some problems such as misjudgment or appearance of a block shape at a boundary between a binary image area and a tone image area. When method B is adopted, an effect caused by misjudgment is not so conspicuous as in method A. However, a difference in density between a misjudged portion and a portion properly judged is created, which leads to incongruity of an image.
Furthermore, it is difficult to tell a drawing in thick solid lines or a flatly blackened portion in a binary image area from a high density portion in a tone image area. If a judgment parameter is adjusted so that they can be distinguished, there will be a flattened density portion in a photograph image. On the other hand, if the judgment parameter is adjusted so that a high density portion of a photograph can be distinguished from a tone image, the density of a thick solid line drawing or a flatly blackened portion will become low.
Therefore, as means for alleviating the problems in methods A and B, Riso Kagaku Corp. has proposed a method (see Japanese Unexamined Patent Publn. No. 8(1996)-51538, hereinafter called "method C") to tell a binary image area from a tone image area, wherein an "edge pixel" (a pixel with a sharp density gradient) and its surrounding are detected, the edge pixel and high density pixels in its surrounding are then judged to be in a binary image area, and, in the case of misjudgment such as the edge pixel having been judged as a contour pixel in a tone image area, an appropriate density conversion curve is selected among a plurality of density conversion curves which interpolate a tone image density conversion curve and a binary image density conversion curve in order not to create incongruity of an image as the distance between a binary image pixel to the edge pixel becomes larger.
In method C, two conditions, that is, the target pixel is close to the edge pixel and is in high density, should be met for the target pixel to be judged as a pixel in a binary image area. However, binary image areas wherein density varies in a background or between characters may coexist on an original, such as in the case where hand written letters lie next to a piece of newspaper cut and pasted on an original, or a document printed by a laser beam printer accompanies a seal (letters in red seal-ink) on the document. In such a case, as well as in the case where tone image areas such as photographs coexist on an original with the background or letters, faint letters (especially letters in pencil or red seal-ink) may be misjudged as a tone image area rather than a binary image area, since the density gradient thereof is not very sharp and the density is low. As a result, the faint letters may disappear on print.